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Gu 16 


Bulletin No. 16 M. M, LEIGHTON December 1, 1920, 
COMMONVEALTH OF PENNSYLVANIA 


DEPARTMENT OF INTERNAL APPAIRS 
James F. Woodward, Secretary 





BUREAU OF TOPOGRAPHIC ND GEOLOGICAL SURVEY 
George H. Ashley, State Geologist 








aes 


GEOLOGY OF OIL AND GAS IN RELATION TO COAL. 
4g 
George H. .shley 


There has long existed an idea that oil and gas are Somehow re- 

ted to coal if, indeed, not derived from coal. The occurrence of 

il, gaS, and coal.in the same areas in many parts of the world, 
20upled with the fact that gas is given off from coal in mines and 
that both oil and gag can be distilled from cosl, is largely resvonsi- 
ble for this idea. Granting thet this geographic relationship is 
ourely fortuitous; the more serious question arises: Just whet are the 
relations of coal, oil and gas in their origin, character and occur- 
rence* 


all are hydrocarbons, that is, consist of carbon and hydrogen — 
with some oxygen and nitrogen and possibly other substances, If coil 
be heated in a retort, oil and gas are given off which have many of 
the properties of oil and gas as found in nature and. separable into 
much the same series of primary compounds. 


Furthermore, our common theories of the origin of ali three 
deseribe them as derived from the remains of orgenic material. Mr. 
Reinhardt Thiessen hes shown that coel is made up of remnants of land 
and water plants that may have accumulated originaliy as a peaty 
deposit, Similar studies on the "oil shales" of Colorado by C. A. 
Davis show thet those shales consist in the main of plankton mixed 
with mud. Plankton is the gelatinous material composed of the remains 
of water weeds and smll lske animals thet accumulates at the bottom 
of lakes. 


Chemical and microscopic studies of cannel coal indicate that it 
lies about half way between bituminous coal and oil Shale. It, too, 
4~ 2 water deposit but apvears to differ from the shale in that while 












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the shale contains principally the remains of gelatinous water piants, 
cannel coal contnins mainly the srores and decomposition products of 
the spores of ferns and other plants such as made up the bituminous 
coals. 


It would seem, therefore, that bituminous é¢oc2l, cannel coal, and 
Oil Shale differ in that bituminous co2l1 is deriveé from the remains 
of land or water plants, with more or less woody parts, which grew 
where the coal is found; cannel coal is derived mainly from the spores 
of these sane land and water plants floated to the place where the 
cannel coal is now found, usually lagoons within the coal swamp; and 
Oil shale derived its bituminous materiel from water plants and small 
animals accumulated on the bottom in open bodies of water. 


ODL end eas ldsrter. trom /coalvin one .imrortant particular,’ Goal: 
SU yo mneros it was puts (OL) and eas as a rule,’ do notvor, i at least, 
6 Suppose that they do not because they can move through the rocks 
aa we believe that they were not deposited originally in the sands 
nere they ere commonly found today, That water can travel long dis- 
tances through rocks, possibly hundreds of miles, is proved by our 
“nowledge of artesian waters. That gas can and does travel throu: 
the rocks at least a few miles, is' well established by a study of gas 
occurrences and rock pressures in the gas fields. That oil will 
travel short distances is very well established. That it will travel 
long distances seems'a safe inference from our knowledge of the move- 
ment of water in the rocks. 
Then comes the all important problem: How do oil and gas travel? 

Gas, we know, travels in two ways: by its own expansion, from high 
pressure to low pressure areas; and by being pushed ahead of Licuids 
advancing with superior pressure. Oil may be said to travel in four 
* ways: First, it mns down hill under the attraction of gravity. 
Second ,- it will move forward under the influence of a gas exerting an 
unbalanced pressure on one Side. Third, it will rise to the surface 
of a heavier liquid and ride on toy of such a licuid if moving. 
Fourth, it will move through minute spaces by capillary attraction, 
as when oil rises in the wick of a lamp. Apparently ell four of these 
conditions exist in nature. 


To get more clearly in mind the conditions uncer which oil and 
gas accumulate in the rocks - for it is this accumulation which has 
wractical interest - let us recall: Tirst, tmt the beds of reeks 
aiffer in porosity and thet the same bed differs greatly in porosity 
Within itself. Second, that practically no rock lies exactly flat »vut 
all are tilted or folded in various degrees. The terme "monoclins", 

Wad oa Gul amore nel ui emer Rod ete Mn oe dome My ards eter ase 

Arevtamiilarw Third,» that aALimay' be doubted iif’ oll wilk. commonly ron 
DUiOL, Sx rock exceny Under Preasure Ol elthom gas) air) or artetian 
water. This is clearly shown where supposecly exhausted oil fields 
are revived by admitting gas, air or water under pressure. 

Experiments indicate thet the producing oil sands have & porovr:: 
of one-tenth to one-fourth of the volume of the rock, with an averare. 
of around one-fifth. Therefore, a little cifference in the pore sp2c 
may determine if a.rock is, productive or dry. In many fields the por: 


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Space seems to be the principal or only factor in determinine where 
O11 and gas will be found, In others, it is only one of the factors. 


Oil moty occur in association with water or in a “dry" or “water 
«ree"™ sand. It is probably always associated with some gas, though 
625 is not always accompanied by recoverable oil. 


‘here water, oil, and gas occur in the same stratum in inter- 
communicating space, they arrange themselves in the rock in the order 
citec, from the bottom up. Thus the gas is found in the highest part 
of the rock reservoir or only on the enticlines. The oi1 is found ox 
the flanks of the enticlines md the water in the synclines. This 
Simple theoretical arrangement, however, seldom holds sood, as will be 
evident to any one who studies the structural relations of the oil and 
gas pools of southwestern Permsylvania. Inthe first place, beyond 
3,000 feet below the surface, there is an increasing tendency of the 
Bandstones to be water free. The Sone sandstone that contnrins water 
in easter Greene County, Pennsylvania, mav be dry in western Greene 
County where it is several hundred feet deever. gain, few or none of 
the anticlines are flat crested but rather "camel backed", not only 
rising irregularly into domes and saddles mut heaving a general rise 
alon= their axes toward one end. Thus, practically all of the anti- 
clines and synclines. of westem Penmnsvivania have « gener2l rise from 
the southwest toward the northeast, so that the tov of an anticline 
at one point may be much lower than the bottom of the syncline either 
Side at points a few miles to the northeast. Under these conditions, 

if a bed of sandstone is partly fillec with weter and the top of this 
water forms a level, as it will if free to move, it is obvious thet 
water level will be on top of the anticline -t cne noint ond in the 
nov som of the adjoining synclines a few miles northeast. It follows 
woo At Chere TS oll vine on the; water, this.oid will Lie on) the 
anticline in one area, {with g2zs, if present, fcrther up the same 
anticline) and in the syncline farther northeast where the synclines 
have risen above the water level. 


These statements give e Slight idea of the difficvity of pre- 
digting the presence of oil and gas in any aren witnout a very wide 
ane commorehensive study of all of the conditions (not includcinz the 
nore svace of the xwocks, which cannot even be esuessed at from the 
guritece). | Por example, inia given stratum or "sand" in eastern) Crean 
County, oi11 i185 found above water. How may we predicate the vresenc 
Or ‘either Oil or (gas farthéy. to the west. where the Sand is deopor one 
contains no water? We do find both oil and gas in the water-free 
Sands where deever farther to tho west. How did they get below the 
water? Has the water been driven pest them out of the deeper rock or 
is: the water working its way down, from the surface? In places water 
and oil are »pumped together from the wells as though in those »loces 
the water were passing the oil either up or down as the case might be, 
ané had not yet reached the naturel order of arrangement with rererence 
to weight. 


These sre a few of the problems that the new Geological Survey 
has set itself to try to work out for Permsylvania and it is hoping 
to reach some definite, helpful conclusions before the oil and gas 
fields of the State heve become only a memory. The hopeful element 


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in the problem is our belief that the oil fields are yet a long way 
from exhaustion if a method can be found to get from the sands all 
the oil not yet pumped out. For example, 2 sand with a pore space of 
1/5 (20%) has a capacity of about 1,550 barrels of oil per acre foot. 
The Bradford field in MeKean County is estimated to contain 85,000 
acres and the sand is estimated to Mave an average productive thick- 
ness of 45 feet. Assuming 1,000 barrels to the acre foot (for the 
Sand is closer grained at Bradford than most sands) gives 2 total 
capacity of 3,825,000,000 barrels, The estimated total production to 
date is 260,000,000 barrels or about 1/16 of the total. ‘Je do not, 
of course, know the exact porosity of the Bradford sand nor that the 
juve Space in that sand is full of oil. The figures, I .wever, show 
brat even if the pore space is over estimeted and the thickness of 

1¢@ productive sand also over estimated, there still is room for the 
oroduction of many times as much oil as Ims yet been obtained from 
uhat field. Whether the same condition is true in other fields where 
the sand is more open grained, is not yet certain, but the evidence 
at hand points to the fact that the supposed exhaustion of most of 
the oil fields is really the exhaustion of the gas or water pressure 
thut previously forced the oil into the wells. The problem is to 
get out the oil remaining. 


Lest anyone think that because of these difficulties the possidle 
presence of oil or gas cannot be predicted, let me hasten to state 
that, given time for investigation, the geologist can be of very . 
great service to the oil and gas industry. He can never reach 100% 
efficiency, but consitering the cost of oil and gas wells, if he can 
reduce the failures from one in three ‘to one in’five, his services 
certainly are worth while. As evidence of this, Mr. F. G. Clapp has 
estimated that in nine townships of northern Oklahoma success in 
driiiing twerveased from 60% before geologic advice was sought to 87% 
following geologic advice. Dorsey Hager has show thet of the new 
pools opened in Oklahome between 1913 mad 1917, 70% were found on 
geologic advice. Under scientific direction one wild cat well out 
of three found oil as against one out of one hundred and fifty 
drilled in the usual haphazard way. Hager also finds that out of 
seventy five most important oil pools in Oklshome and Kansas dis-~ 
covered during those years, all but four were on well defined 
structures such as domes, anticlines and terrseces, 


Unfortunately, it is ‘not possible to rench the same success i. 
gost fields. For exemple, in parts of Texas and Louisiana, the 
surface geology bears little rclation to the underground geology ane. 
the geologist can be of little help except es he interprets drilling 
as it progresses and on that basis guides new drilling, 


In the Appalachian oil and gas field, which at least is nearer 
home, it may be noted: first, that much of the area lies within the 
Appalachian coal field and tmt within the cool field oil is confined 
to those areas where the coal is fairly high in volatile matter, 
having a fuel ratio of 2,orsless. 4 possible cause for this is dis- 
covered when we remember thet all the coal was originally high in 
volatile matter ond that the low volatile coals have had port of the 
volatile matter driven off by the forces thot foldcd the rocks; and if 
we note also thrt the biack shmles of the same region, when distillcd 


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for oil, yield no-oil in the areca of the anthracite coal, little oil 
in the resion of the high carbon coals and much oil in the regions cf 
the high volatile coals. Evidently the lack of oil in the areas of 
high carbon coals is because the oil has been driven off or distilled 
into gas, 


If we recall that coal, when distilled, will yield oil if heater 
to a moderate temperature, Say less than 7509F., and gas, if heated 
to a high temperature, say 1200°P. , we might expect t0 find gas in 
areas of moderately high carbon coals and so we do. Thus if pros- 
pecting within the cel field, we might anticipate finding gas but 
not oil where the coal has a fuel ratio between 2 and 3.5, oil and 
gas if the fuel ratio is below 2, and neither oil nor gas if the fuel 
ratio is much above 3.5. 


Im the second place, oi] and gas are found so commonly in asso- 
ciation with folds in the rocks it even has been suggested thot the 
oil or gas is distilled only in the rocks where found and by the 
folding in those places. 


It is more likely that the folds have served as places of accunu- 
“lation because they interrupt the movement of oil on water.: Obvious- 
ly oil working its way down 2 slope which meets no obstacle, will 
ultimately land in fhe adjoinine (eyneline. Or, af being carriod up 
a& slope under the pressure of accumulating watcr, it will ultimtely 
ed) On tOp, OF : the Sere net, ig, however, in moving down,a lons 
2ope a flat place or BLiphtiy reversed din .is encountered, some, oc 
oil is apt to lodge there; or, if water accompanies the oil, the 
ter may lodge on the fint or bench and :form a Seal that holds the 
IL 2 SB beeeHs TSO Phat ne ad Moyo oe found not on the bench but -t the 
foot of the slope above the. bench, --Ageain,.if is found that in plinccs 
on. long even monoc lines there aro folda. mirage to bhe dio makine 
what on 2 hillside we would call noses and ravines. Experience has 
Shown that many such structural "noses" or "ravines" are the site of 
zood oil pools. 


Such facts suggest that the most likely explanation Of ACCU. tes 
tons of ol] and gas in structures on the fisnks of enticlines or on 
monoclines, is that: the folding at these points haesslightly chaneed 
BNe \MiNULE SvIactire of the woek. A careful study ‘ot bent slabs of 
rock in grave yards or in buildings reveals a certain smount of almost 
microscopic fracturing where the rock has stretched, even though the 
peut is so slight ss to be hardly noticeable.’ ,It would seom quite 

Ossible. therefore’. that such Lloeni folding as I have mentioned 
faeht open up the rock in one area, and tend to seal it in another 
ates Many LUpinie \oil travels bys onpiliarity, such’ a change in’ the 
character of the rock would tend to retain the oil and create 2 pool 
at the fold, 


Discussion is omitted of the one hundreec and one finer »oints 
of the subject. such as the fnct that in Ponnsylvanio. the structure 
of the oil md gas sands does not quite agree with the surfrce 
structure, due principally to on unconformity at the base of the 
Pottsville; the cffect of unsymmetrical folding of the snticlines; 
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In conclusion, if asked to examine .ny nrea for oil and gas, the 
following srocedure ie suseested: 2. Study the fuel ratio o 
or tie oil content of black sh-les if they ore present. 2. Prepcrie 
J accurate a structure cantour map of a surface key rock as con- 
‘itions will permit (if in a region of gently dipping monocline, ten 
root contours should be obtained, using a plane table).: 3. Preocxre 

structure contour map of each oil-or gas-bearing sand, using “con- 
vergence sheets", that is, sSheots Showing the varying intervals from 
that Sand to the surince key rock, 4. Study carefully locel pecu- 
112 ‘rities - water pre aats porosity or grain of "sand"; Tlow cui 


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of oil welis, rock pressure of gas, variations a rock vressure in 
different wells as Rae ting “lay” of the pool ea at various times 


Ae SNeOVexrting The Lite or the pool. 


With these data on hand, you are ready to "go to it". 


inn 


077640826 


